Christopher L. Morrow's mention of asymmetric routing for multihomed customers is more to the point, but if we can solve this for all those single homed dial, cable and ADSL end-users and not for multihomed networks, I'll be very happy.
Sorry to throw yet another insect into the topical remedy (fly in the ointment), but, I happen to look alot like a single homed ADSL end user at certain levels, but, I'm multihomed. I'd be very annoyed if my ISP started blocking things just because my traffic pattern didn't look like what they expect from a single homed customer.
I prefer the latter.So which do you prefer: nobody gets to scan your systems from the outside (including you) or everyone gets to scan your systems from the outside (including you).
If you want to know how TCP is working to a destination, you have to use TCP to test it.
As I mentioned above: this will not impact TCP at all because TCP generates return traffic. I'm sure there are one or two UDP applications out there that don't generate return traffic, but I don't know any. The only problem (except asymmetric routing when multihomed) would be tunnels, but you can simply enable RIP or something else on the tunnel to make sure it's used in both directions. Multicast doesn't generate return traffic so this would only apply to unicast destinations.
But, TCP to a port that isn't listening (or several ports that aren't listening) _ARE_ what you are talking about blocking. This is not a good idea.
Scans by themselves certainly aren't inherently dangerous.
It should be possible to have a host generate special "return traffic" that makes sure that stuff that would otherwise be blocked is allowed through.
I don't think it's desirable or appropriate to have everyone re-engineer their hosts to allow monitoring and external validation scans to get around your scheme for turning off services ISPs should be providing.
Owen
